Unwind to use ef reverse iterator

[AskAlexSharov]

[] Unwind: does ht.IdxRange(k, int(txNumUnwindTo)-1, 0, order.Desc, -1, rwTx) for each key. Probably can fix it by using HistoryRange() which returns 3 things: key/value/step

[] Domain.Prune does full-scan - probably can fix it by use of HistoryRange() #10480

If it's still impossible, then:
[x] then implement EliasFano.ReversIterator #10395 and after it - see where will be the bottleneck (in unwind).
[] to use Reverse iterator